redhat man page for catch

catch(n)						       Tcl Built-In Commands							  catch(n)

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NAME
       catch - Evaluate script and trap exceptional returns

SYNOPSIS
       catch script ?varName?
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DESCRIPTION
       The  catch command may be used to prevent errors from aborting command interpretation.  Catch calls the Tcl interpreter recursively to exe-
       cute script, and always returns without raising an error, regardless of any errors that might occur while executing script.

       If script raises an error, catch will return a non-zero integer value corresponding to one of the exceptional return codes (see	tcl.h  for
       the  definitions of code values).  If the varName argument is given, then the variable it names is set to the error message from interpret-
       ing script.

       If script does not raise an error, catch will return 0 (TCL_OK) and set the variable to the value returned from script.

       Note that catch catches all exceptions, including those generated by break and continue as well as errors.  The only errors  that  are  not
       caught  are  syntax  errors found when the script is compiled.  This is because the catch command only catches errors during runtime.  When
       the catch statement is compiled, the script is compiled as well and any syntax errors will generate a Tcl error.

EXAMPLES
       The catch command may be used in an if to branch based on the success of a script.

	      if { [catch {open $someFile w} fid] } {
		  puts stderr "Could not open $someFile for writing
$fid"
		  exit 1
	      }
       The catch command will not catch compiled syntax errors.  The first time proc foo is called, the body will be compiled and a Tcl error will
       be generated.

	      proc foo {} {
		  catch {expr {1 +- }}
	      }

KEYWORDS
       catch, error

Tcl									8.0								  catch(n)